The market demand for solar energy collection systems in the form of photovoltaic cells (PV) is growing in excess of 25% per year globally due to factors including higher oil prices and government policies addressing such environmental issues as global warming. The dominant substrate material for PV is silicon, which accounts for about 90% of installed commercial units at the present time. A serious shortcoming in the silicon-based PV value chain, however, is that there is presently no direct method of producing PV-grade polycrystalline silicon (PV-Si) at competitive prices. The main reason for this situation is that, historically, the PV industry relied mostly on scrap silicon material that was recycled from the microelectronics industry. Recently, the global demand for PV-Si has outstripped the supply of recycled electronic-grade silicon (REG-Si) and the expectation is that this source of silicon will no longer be able to meet the demand from the PV industry.
Many PV manufacturers are now considering direct purchase of electronic-grade silicon (EG-Si), which is also in tight supply, but whose price is as much as 10 times higher than the historical average price of REG-Si. The higher price of EG-Si is mainly due to the complexity and high capital cost of the trichlorosilane and silane processes that dominate this industry at the present time. In many cases, the EG-Si producers are also forward integrated into the microelectronics value chain and so these processes are optimized for that end-user market. What is required for the PV industry is a process that is simpler, more economical, and safer to operate than the dominant EG-Si processes.
In U.S. Pat. Nos. 6,712,908 and 6,468,886, Wang et al. disclose a three-step process for the production of PV- and EG-silicon. In the first step, impure metallurgical-grade silicon (MG-Si) is reacted with iodine at a temperature, (T<900° C.), which favors the formation of silicon tetraiodide, SiI4. Sufficient SiI4 is then produced in this fashion to fill a holding tank. Once the required amount of SiI4 is produced it is then recycled to the initial reactor stage where the temperature has been increased to above 1200° C. and the SiI4 reacts with MG-Si to produce substantial quantities of an unstable silicon diiodide vapor compound, SiI2. The SiI2 is transported by natural convection to a cooler region of a “cold-wall” reactor where it decomposes and deposits as polycrystalline silicon on solid substrates that can be inert or high-purity silicon rods.
However, there exist a number of problems associated with the teachings of this invention and others in the prior art, that, taken together, prevent the realization of a scalable and economical method for the production of PV- and EG-silicon. These shortcomings are described in detail below.
1. The use of a “cold-wall” vessel for the reaction and deposition of silicon is critical to the method and apparatus of the invention disclosed by Wang et al. Yet this leads to poor control of the spatial distribution of silicon deposition due to three factors: 1) the SiI2 decomposition reaction that forms silicon is a function of temperature; 2) SiI2 readily decomposes to form solid silicon in the vapor phase without the requirement of a solid substrate; 3) the temperature gradient between the SiI2 formation zone (i.e., T˜1200° C.) and wall region (i.e., T=200-700° C.) of the reactor is at least 500° C. Furthermore, as the product vapors that are saturated with SiI2 form in the reactor bottom, some of the vapor travels toward the cooler walls and thereby creates a thermodynamic driving force for fine silicon powder nucleation within the vapor phase. The quantity of silicon powder may be anywhere from 10-50% of the total silicon produced at any given time. This silicon powder will be produced homogeneously and will be entrained along with the liquid silicon tetraiodide, SiI4, stream as it is injected into a batch distillation column. As there are no provisions for the separation of the entrained silicon fines, the distillation column operation will be compromised and the process will need to be shutdown for frequent cleaning thereby making the process less viable. Also, the silicon thus produced is very fine and not generally in a usable form due to its tendency to oxidize with air at ambient conditions in the facilities of the end-user ingot and wafer manufacturers. To summarize, the interaction of the three factors described above results in the production of a substantial amount of silicon product that is both unsuitable for sale and difficult to remove from the process, thereby reducing the economic viability of Wang et al.
2. More than half of the weight of impurities in MG-Si typically consists of Fe atoms. While Fe reacts with SiI4 to form FeI2 vapor in the lower part of the cold-wall chamber at temperatures of about 1250° C., as the vapor temperature decreases to 700-800° C. near the cold-wall, the Fe is converted to solid FeSi. Due to the poor control of temperature in the cold-wall reactor, it is likely that a majority of Fe atoms will be entrained as solids within the liquid stream of silicon tetraiodide as it is directed to the distillation unit.
Again, as in the case of gas-phase silicon formation, these impurities affect the operation of the distillation column by contaminating the recycle stream and plugging of distillation column internals. As the teachings of Wang et al. do not accommodate the removal of these impurities, they tend to build up in the process and will be recycled back into the cold-wall reactor where they substantially reduce the overall efficiency of purification.
3. The cold-wall reactor is operated as a natural-convection driven system and this leads to the formation of a vapor cloud located near the uppermost region of the reactor. Because of the existence of this vapor cloud, the preferential removal of Boron (B) and Phosphorous (P) on the top section of the reactor does not occur as there is no provision made for preferentially removing the iodides of these elements from the other predominant compounds in the vapor cloud such as silicon tetraiodide, iodine, and other impurities. Also, any elemental silicon or silicon iodide that is inadvertently removed from this section of the reactor is not recoverable by the teachings of Wang et al.
4. Wang et al. teaches a method and apparatus for purifying silicon tetraiodide in a distillation column that is operated in a batch mode with the input SiI4 stream introduced in the bottom section. This type of system is referred to as a “batch distillation without reflux”. In this mode of operation, the level of purification is generally not very good and certainly cannot meet the 10,000-to-1 or more reduction of impurities levels in SiI4 required for the process to be effective in the recycle loop. Furthermore, large-scale use of batch distillation is not generally practiced because of the high costs associated with startup and shutdown operations.
5. Iodine raw material added to the process is typically more expensive than the MG-Si. Therefore, the need to minimize the use of iodine within the process and to recover iodine from impurity output streams is an important part of ensuring an economically scalable process. The method and apparatus of Wang et al. does not teach how to recover iodine from the solid and liquid iodides formed (e.g., FeI2 and AlI3). Furthermore, Wang et al. does not show how to minimize the use of iodine within the process to minimize initial capital and operating costs for the commercial plant.
6. The method and apparatus of Wang et al. assumes that there is no free-iodine (i.e., I or I2) left in the system once the second stage of operation is started and silicon tetraiodide is recycled into the cold-wall reactor. Thermodynamic calculations reveal, however, that between 1100-1300° C. the reaction between solid Si and SiI4 vapor in the reactor bottom produces the following compounds with the stoichiometry indicated:Si(s)+2.5SiI43.4SiI2+2.3I+0.3I2+0.1SiI3 By neglecting to account for the presence of free iodine, the distillation column design ignores the need to condense, purify, and recycle this expensive raw material as there is no reflux capability on the top of the column.
7. In a commercial process, the iodine raw material will contain impurities that need to be removed. If the source of the iodine is a caliche ore deposit then these impurities are generally water, non-volatile solids, and chloride-bromide compounds. No means to remove these impurities is disclosed in Wang et al.
8. Wang et al. does not provide an economical method for producing EG-Si. Experimental results provided by the teachings, for example, indicate a purity level for B and P of 4 and 7 ppm atomic, respectively, for the case where there is no recycling of purified SiI4. In order to decrease the B and P levels even further to the EG-Si specifications that are in parts per billion will require a recycle ratio of SiI4-to-input MG-Si that is in the range of 100-1,000. This amount of recycling is prohibitively expensive in commercial systems and so a method is needed to substantially reduce the SiI4 recycle ratio and the size of the distillation column to make this chemistry economical versus the competing trichlorosilane and silane methods previously discussed.
9. Natural convection is the primary mode of mass transport in the “cold-wall” reactor. This method of mixing reactants does not lead to high productivity and is generally avoided in chemical process systems in commercial applications because it leads to unnecessarily high capital costs for plant and equipment.
10. There is no means to remove liquid iodide impurities in the batch distillation column that have higher boiling points than CI4.
In sum, the deficiencies in the forgoing invention make it very difficult to economically produce purified silicon on a commercial-scale.
Other related art includes: U.S. Pat. No. 3,006,737 to Moates et al; U.S. Pat. No. 3,020,129 to Herrick; U.S. Pat. No. 4,910,163 to Jain; and U.S. Pat. No. 6,281,098 to Wang et al.
Related publications include: Herrick, C. S. et al., “High-purity Silicon from an Iodide Process Pilot Plant,” J. Electrochem. Soc., Vol. 107, No. 2, February 1960, pp. 111-117; Glang, R. et al., “Silicon”, in The Art and Science of Growing Crystals, John Wiley and Sons, New York, 1963, pp. 80-87; Szekely, G., “Preparation of Pure Silicon by Hydrogen Reduction of Silicon Tetraiodide,” J. Electrochem. Soc., Vol. 104, No. 11, November 1957, pp. 663-66′7; Litton, F. B., et al., “High Purity Silicon,” J. Electrochem. Soc., Vol. 101, No. 6, June 1954, pp. 287-292; Glang R., et al., “Impurity Introduction during Epitaxial Growth of Silicon,” IBM Journal, July 1960, pp. 299-301; and Hillel, R. et al., “Stabilité Thermique et Propriétés Thermodynamiques des Iodures de Phosphore a l'état Condensé et Gaseux,” J. Chimie Physique, Vol. 73, No. 9-10, 1976, pp. 845-848.